I couldn't find any guides online on how to do this so I figured why not make one.
(This is not a guide on the firework rocket itself, but the firework star used to make the rocket.)


The following command is the default format with all the tags included:

/give @p minecraft:firework_star{Explosion:{Type:0b,Trail:0b,Colors:[I;15435844],FadeColors:[I;],Flicker:0b}}

This command will give you a Small Ball, default Orange dye color firework star.

Custom Color:

The number: 15435844 is a decimal number and can be replaced with any decimal number.



You can use this site to view decimal/hexadecimal colors, convert hexadecimal -> decimal or decimal -> hexadecimal, or generate new colors:
https://www.mathsisfun.com/hexadecimal-decimal-colors.html


To add more than one color, insert a comma followed by another decimal number after the first number:
/give @p minecraft:firework_star{Explosion:{Type:0b,Trail:0b,Colors:[I;15435844,4312372],FadeColors:[I;],Flicker:0b}}

(to add more colors just repeat the pattern)

The above command will give an orange and lime firework star. Also, if you run the command, you will notice that the orange and lime colors are now mixed together, creating a new color on the firework star's texture.

Custom Fade Color:

By default the custom Fade Color is disabled, to enable it, insert a decimal number in the FadeColors tag, after the I; :

/give @p minecraft:firework_star{Explosion:{Colors:[I;15435844],FadeColors:[I;11743532]}}



Explosion Type:

To change the explosion type of the firework star you will have to change the number after Type: .

/give @p minecraft:firework_star{Explosion:{Type:0b,Colors:[I;15435844]}}



Warning, Do not use 5b or higher on servers, it will prevent you from joining your world again. (To fix this, get another player/account with perms to put down an active repeating command block that has "clear NAME firework_star" in it and then rejoin on your account until you don't get kicked)


By default if you run the command without the Type tag, it will give you Small Ball type:
/give @p minecraft:firework_star{Explosion:{Colors:[I;15435844]}}


Adding a Trail & Twinkle:

By default the Trail and Twinkle (Flicker) effects are disabled, to enable them change their values from 0b (disabled) to 1b (enabled).

/give @p minecraft:firework_star{Explosion:{Type:0b,Trail:0b,Colors:[I;15435844],FadeColors:[I;],Flicker:0b}}

This will enable both a Trail and a Twinkle (Flicker):

/give @p minecraft:firework_star{Explosion:{Type:0b,Trail:1b,Colors:[I;15435844],FadeColors:[I;],Flicker:1b}}



Hiding the lores (HideFlags):

If you want to hide the Color name, or any of the explosion effects from the items lore, add HideFlags:32 to the end:
/give @p minecraft:firework_star{Explosion:{Type:0b,Trail:0b,Colors:[I;15435844],FadeColors:[I;],Flicker:0b},HideFlags:32}